Because it's a horrible UX paradigm to require. If 'Save As...' had never existed, it wouldn't be unreasonable, but it is (was) a reasonable behavior that every other system I'm aware of has maintained.
This passive versioning thing is quite nice in some aspects, but it's very unknown with most people I've seen. They see a file changed, they assume it's stuck like that now and the original is gone forever. Why would they expect otherwise? Their unsaved file gave no indication of being saved. Every application they open it in, and everyone they email it to, will only get the most-recent version. Even the file-system doesn't tell you when you have multiple versions of something.
This passive versioning thing is quite nice in some aspects, but it's very unknown with most people I've seen. They see a file changed, they assume it's stuck like that now and the original is gone forever. Why would they expect otherwise? Their unsaved file gave no indication of being saved. Every application they open it in, and everyone they email it to, will only get the most-recent version. Even the file-system doesn't tell you when you have multiple versions of something.